Enhanced oil recovery (EOR) processes employ thermal energy to facilitate the recovery of oil, particularly heavy oil, from oil-bearing formations. One particular process widely used in Canada for recovering heavy oil is referred to as steam assisted gravity drainage (SAGD). In a SAGD process, steam is injected into the oil-bearing formation. Generally, several tons of steam is required for each ton of oil recovered. In this process, steam heats the oil in the oil-bearing formation and this reduces the viscosity of the oil and facilitates the flow of oil to a collection area. In this process, however, steam condenses and mixes with the oil to form an oil-water mixture. The mixture of oil and water is pumped to the surface and oil is separated from the water by conventional processes that form what is referred to as produced water.
It is important to treat produced water so that it can be reused. In a SAGD application, the produced water is treated or purified and is directed to a steam generator or boiler that produces steam. In a SAGD application, for example, the complete water cycle includes: (1) injecting the steam into an oil-bearing formation; (2) condensing the steam to heat the oil which results in the condensed steam mixing with the oil to form an oil-water mixture; (3) collecting the oil-water mixture; (4) pumping the oil-water mixture to the surface; (5) separating oil from the oil-water mixture to form the produced water; (6) treating the produced water by removing suspended and dissolved solids to form a feedwater stream for the steam generator or boiler and (7) converting the feedwater into steam that is injected into the oil-bearing formation.
Produced water typically includes significant concentrations of calcium hardness as well as alkalinity, especially when the produced water is mixed with make-up saline water which contains a lot of hardness. The presence of calcium and alkalinity in the produced water often leads to the precipitation of calcium carbonate scaling compounds. In processes that employ evaporators, for example, calcium carbonate scaling can be a serious problem. This causes the calcium carbonate precipitates to scale heat transfer tubes and preheater heating surfaces substantially decreasing the efficiency, and resulting in expensive and time-consuming maintenance.
In the past, calcium carbonate scaling of evaporators and process equipment has been dealt with upstream of the evaporator or process equipment by acidification and degasification. This process reduces the alkalinity concentration in the wastewater stream. However, acidification and degasification have drawbacks. The acid cost can be prohibitive if the wastewater has a high alkalinity concentration. In addition, in some applications where organics are present in the wastewater, the organics precipitate in response to acidification. In other cases, an ion exchange softening unit only is employed to remove hardness. This process typically reduces the concentration of calcium and magnesium in the produced water. When used as the sole means of reducing hardness, ion exchange units also have drawbacks. The use of ion exchange softening adds extra cost and generates a liquid waste stream which often requires treatment. Also, as a practical matter, there is a limitation on the calcium concentration to be treated by ion exchange resin. Another option for dealing with produced water streams having hardness is employing a chelant or chelant agent inside the evaporator or other process equipment. This chelant agent maintains calcium in a soluble form and hence prevents precipitation. The drawback here is that efficiency is limited and chelant agents are expensive.